The present invention relates to pavement sprayers and, more specifically, to movable guards for preventing the accidental spraying of liquid paving compositions.
Roads, parking lots and a variety of other surfaces are often paved or coated by spraying liquid asphalt materials onto a pavement surface at elevated temperatures. As described in U.S. Pat. Nos. 3,891,585, 4,069,182 and 5,297,893, the specifications of which are incorporated herein by reference, one such method involves heating an asphalt-rubber mixture to approximately 400.degree. F. for a time sufficient to form a jellied composition and spraying the composition onto a surface to be paved.
Commercial systems for the spray application of asphalt compositions are well known in the art. They typically consist of a truck equipped with a tank for carrying the composition and maintaining it at an elevated temperature, a pump for pressurizing the composition, and hoses for transporting the heated and pressurized composition to a series of spray nozzles. The nozzles are mounted to a distributor bar on the rear of the truck and are positioned on the bar to spray the asphalt mixture onto the ground in a preselected pattern as the truck moves.
Because the width of the required spray pattern can vary between jobs, systems for the spray application of asphalt pavement materials often use a combination of distributor bar segments which together exceed the width of the truck and are separately controlled by an operator. In one common configuration, a stationary "main" portion of the distributor bar extends substantially across the width of the truck. A pair of "secondary" outer portions are then mounted to the ends of the main portion so that in an operative position they line up with it. This enables wide strips of pavement to be sprayed by supplying liquid to all portions of the bar, and narrower strips to be sprayed by interrupting the flow to one or more of the outer portions.
The outer portions of the distributor bar are typically mounted to the truck for upward pivotal movement from a horizontal operative position to a substantially vertical retracted position. This reduces the effective width of the truck as it travels between jobs or is used to spray narrower strips of pavement. Unfortunately, however, the nozzles on the outer portions of the bar face outwardly from the truck in the retracted position. In order to avoid harm to bystanders or nearby property, it is important to cut off the flow of paving material to the outer portions when they are in their retracted positions.
Prior distributor trucks sometimes use mercury switches and pneumatic valves to stop the flow of liquid to outer portions of the bar when the outer portions are in their retracted positions. Each mercury switch is oriented to close its contacts only when an associated portion of the bar is in its operative position. The switch acts through a pneumatic actuator to operate one or more fluid valves controlling the flow of heated pavement material to a plurality of nozzles of the bar. When the contacts close, the valves allow liquid to flow to the nozzles on a given outer portion of the distributor bar. Conversely, the contacts open when the outer portion moves to its retracted (vertical) position, deenergizing the pneumatic actuator and the fluid valve. This closes the fluid valve, preventing liquid from being sprayed from the affected nozzles.
Unfortunately, mercury switches, pneumatic valves and electrical wiring can all fail over time, endangering people and property in the area of the distributor bar. This is particularly true where, as in prior systems, a fluid control circuit is activated by switching to ground potential. A short circuit upstream of the switch can then cause spray valves to be activated erroneously.
In view of the harm that can result from failure of conventional fluid control circuits, especially when hot liquids are involved, it is desirable to eliminate any chance of inadvertently spraying liquid while the outer portions of the bar are in their retracted positions.